Hemicellulose hydrolysis digestibility

The effectiveness of dilute acid hydrolysis as a pretreatment has been verified experimentally. Researchers at NREL have characterized the susceptibility of a variety of short rotation woody and herbaceous crops and agricultural residues upon dilute acid pretreatment [17,38-41]. What is more interesting is that the maximum digestibility usually coincides with complete hemicellulose removal. The dilute acid treatment of biomass aimed at hemicellulose hydrolysis has since become a widely accepted pretreatment method for enzymatic hydrolysis [16,42-44]. 

Unconventional Relationships for Hemicellulose Hydrolysis and Subsequent Cellulose Digestion... 

Figure 6. Effect ofxylan removal (a) and lignin removal (b) on enzymatic digestibility of cellulose produced by hemicellulose hydrolysis of com stover at the conditions noted.

Little information is available on the enzymatic hydrolysis of xylan. The kinetics of breakdown of xylan are reported.128 Hemicellulose from linden wood is said to be extensively hydrolyzed124 by the digestive juices of Helix pomatia. Xylanase128 of Aspergillus niger, while active on xylan between pH 2-8, has its optimum at pH 4.0. 

Most often, the rates for feedstock destruction in anaerobic digestion systems are based upon biogas production or reduction of total solids (TS) or volatile solids (VS) added to the system. Available data for analyses conducted on the specific polymers in the anaerobic digester feed are summarized in Table II. The information indicates a rapid rate of hydrolysis for hemicellulose and lipids. The rates and extent of cellulose degradation vary dramatically and are different with respect to the MSW feedstock based on the source and processing of the paper and cardboard products (42). Rates for protein hydrolysis are particularly difficult to accurately determine due the biotransformation of feed protein into microbial biomass, which is representative of protein in the effluent of the anaerobic digestion system. 

Many favor dilute sulfuric acid pretreatment because both high hemicellulose recovery and good cellulose digestibility can be achieved (6-8). Moreover, most of the soluble sugars from dilute-acid pretreatment are released as monomers that can be readily fermented to ethanol by recombinant organisms (9,10). Pretreatment with just hot water or steam, termed uncatalyzed hydrolysis or autohydrolysis, eliminates chemical additives, lowers the cost of materials of construction, and generates less waste, but hemicellulose and cellulose yields from batch systems are limited. 

Many factors influence the reactivity and digestibility of the cellulose fractions of lignocellulose materials. These factors include Hgnin and hemicellulose content, crystalhnity of cellulose, and the porosity of the biomass materials. Pretreatment of Hgnocellulosic materials prior to utiHzation is a necessary element in biomass-to-ethanol conversion processes. The objective of the pretreatment is to render biomass materials more accessible to either chemical or enzymatic hydrolysis for efficient product generation. The goals of the pretreatment are ... 

Xylanase supplementation in enzymatic hydrolysis is effective for the substrates with high xylan content. Xylanase supplementation not only increases the xylan digestibility of treated biomass but also the glucan digestibility. Xylanase reduces the hindrance caused by resilient hemicellulose layer on the cellulose microfibril, thus, improving cellulase accessibility to the cellulosic part. 

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